1. Field of the Invention
This invention relates to a method and apparatus for concentrating an aqueous polytetrafluoroethylene (hereinafter "PTFE") emulsion. Particularly, it relates to a rapid method of concentrating aqueous PTFE emulsion without the formation of any kind of coagulating material and an apparatus for such method.
2. Description of the Prior Art
Aqueous PTFE emulsion is well known as a raw material for glossy agents, films, metal coatings, and ceramic coatings, etc. The aqueous PTFE emulsion to be commercially interesting has 60% of polymeric solids content, while the solid polymer content of PTFE emulsion prepared by emulsion polymerization is below 30% by weight polymeric solids. Therefore, in order to increase the solids content, it is necessary to use a process for concentrating aqueous emulsions prepared by emulsion polymerization reactions. Traditional concentration methods of emulsions are carried out by sedimentation, vaporization, and adsorption methods using nonionic or cationic surfactants. The sedimentation method concentrates emulsions by precipitating solid polymer by means of a non-ionic or anionic surface active agent. The sedimentation method using ionic surfactant exhibits the phenomenon that the flocculated particles, via peptizing of the inactive dispersing agent, become emulsified again when the excess water in the upper layer is eliminated after the sedimentation (U.S. Pat. No. 2,478,229). Furthermore, concentration methods using a surfactant require diluting the emulsion in the early stage in order to prevent the flocculation phenomena of the emulsion using the sedimentation method and ultimately require a long concentration time. Also, the method using a nonionic surfactant exhibiting the phenomenon of solubility inversion above certain temperatures has been carried out for the concentration of aqueous PTFE emulsions. It is necessary to concentrate the emulsion by heating to 50-80.degree. C., by dissolving the surfactant in the emulsion, and by adding a small amount of electrolytes (Canadian Patent No. 536,455). As the concentration does not occur without adding the electrolyte or heating in this method, because of the stability of emulsion, it is impossible to obtain an emulsion with high solids concentration. This method, in fact, has difficulty in controlling the amount of electrolytes, because of the concentration of electrolytes being changed in very narrow temperature range. There are also concentration methods by absorbing water using water-insoluble adsorption agents, such as silica gel or ion exchange resins (U.S. Pat. No. 3,668,167). It is also known to carry out the concentration of the emulsion by vaporization of water under reduced pressure after stabilizing the emulsion; however, it is difficult to avoid the formation of foams and overheating (U.S. Pat. No. 3,316,201). The methods described above are very hard to apply to emulsions containing 30% solids content, but also are very difficult to avoid flocculation of the emulsion. Furthermore, concentration methods using a large quantity of surfactants make it necessary to recover surfactants contained in water, and need post-treatment processes for separating or treating wastes after concentration. For conducting this process, much energy is required, and environmental contamination is a problem. In addition, there are other problems such that the rate of treating PTFE is low and fluorinated surfactants inherently contained in the PTFE raw emulsion cannot be recovered.